The present invention broadly relates to a process and electrolyte for imparting improved corrosion protection to chromium substrates, and more particularly, for improving the corrosion resistance of chromium plated articles having a nickel plating underlying the chromium overplate. More specifically, the present invention is particularly applicable for improving the corrosion resistance of nickel plated substrates having an overlying chromium plating deposited from trivalent chromium electrolytes.
A variety of compositions and processes have heretofore been used or proposed for use to impart improved corrosion resistance to chromium plated substrates to retard the formation of rust spots when exposed to corrosive environments. The imposition of more stringent requirements on the corrosion resistance of such chromium plated substrates such as used as components of automobiles in which they are subjected to cyclical corrosive environments has occasioned the need for still further improvements in the corrosion resistance of such substrates. Conventionally, such substrates comprise a ferrous base metal such as steel which is provided with a semi-bright to bright nickel plating thereover followed by a relatively thin chromium overplate. Chromium platings electrolytically deposited from conventional hexavalent chromium electrolytes are generally characterized as passive and nonporous and provide satisfactory corrosion protection in many instances. Because waste effluents containing residual hexavalent chromium ions have become disfavored because of environmental considerations necessitating stringent waste treatment, chromium electrolytes have been commercially introduced in which the chromium constituent is substantially all in the trivalent state. It has been observed that chromium platings electrodeposited from such trivalent chromium electrolytes do not effectively passivate the surface as well as chromium deposits from hexavalent chromium electrolytes and the trivalent chromium electroplated deposits also have a tendency to be porous. Accordingly, nickel-plated substrates having an overlying chromium plating electrodeposited from a trivalent chromium electrolyte have been found to be inferior in corrosion resistance to similar substrates plated with a hexavalent chromium electrolyte.
There has, accordingly, been a need for an improved composition and process for the electrolytic post treatment of chromium-plated substrates which enhances the corrosion resistance thereof thereby substantially increasing the useful operating life of the substrate in spite of exposure to corrosive environmental conditions. While the benefits of the present invention are particularly pronounced in connection with chromium platings deposited from trivalent chromium electrolytes, beneficial results are also achieved employing the present invention on conventional chromium plating electrodeposited from hexavalent chromium electrolytes.
Compositions and processes of the types heretofore known have primarily focused on the improvement in corrosion resistance of various metallic substrates including those plated with chromium from a hexavalent chromium electrolyte. For example, U.S. Pat. No. 2,314,341 has suggested a method and composition for improving the corrosion resistance of magnesium substrates employing an aqueous alkaline solution in which the part is anodically charged or is connected to an alternating current source and the solution contains a metal ion capable of forming a compound with the magnesium substrate under the conditions of electrolysis including chromates, dichromates, permanganates, borates, silicates, sulfates, fluorides, phosphates and molybdates.
U.S. Pat. No. 2,746,915 discloses an electrolytic treatment of hexavalent chromium-plated substrates employing an aqueous acidic solution containing hexavalent chromium compounds but which are devoid of any buffering agents to enhance the clarity of the protective film produced in accordance with the practice of the present invention. U.S. Pat. No. 2,755,242 discloses a treatment for chromium-plated aluminum in which the article is anodically electrified in an aqueous solution containing 1 gram per liter of chromic acid. U.S. Pat. No. 3,574,069 discloses a process for applying a dual chromium electroplating from hexavalent chromium electrolytes on a substrate in which the second electrolyte contains a silicofluoride compound as an essential constituent. U.S. Pat. No. 3,755,091 discloses a process for electrochemically treating a chromium-plated ferrous metal strip in which a dual chromium plating is applied employing a hexavalent chromium electrolyte with an optional intervening nonelectrolytic hexavalent chromium immersion step. U.S. Pat. No. 3,928,157 describes a post treatment of a nickel-plated-chromium-plated steel substrate employing an anodic heat treatment followed by an electrolytic deposition of a synthetic polyamino resin dispersed in an aqueous solution. U.S. Pat. No. 3,966,570 describes a method of electrolytically post-treating of a chromium plated steel strip employing a hexavalent chromium electrolyte by first subjecting the chromium-plated strip to an anodic treatment in a solution containing sodium dichromate in combination with organic acids and, optionally, followed by a further cathodic electrolytic treatment in the same post-treating solution. U.S. Pat. No. 4,007,099 relates to a cathodic post treatment of a chromium plating to effect the formation of micropores in the chromium layer deposited from a hexavalent chromium electrolyte to protect the underlying substrate from electrolytic attack.
The improved process and composition of the present invention provides benefits and advantages by providing an unexpected increase in the corrosion resistance of chromium substrates, particularly chromium electro deposits deposited from a trivalent chromium electrolyte thereby substantially increasing the versatility of use of such post-treated articles as well as enhancing their corrosion resistance and decorative appearance.